The present invention generally relates to strain wave gear systems and in further aspects to sealing and lubricating systems useable therein as shown and described.
Strain wave gearing has been successfully used in industrial, medical, aerospace and defense fields. Generally, strain wave gearing functions by attaching an elliptical member to the system input, the elliptical member forms an external gear, known as a flexspline, in a shape such that it engages the internally toothed outer housing 180 degrees apart and having clearance between the gears 90 degrees from each engagement. As the input spins the elliptical member, the external teeth engage an internally toothed outer member commonly known as a circular spline. The externally toothed gear has less teeth than the internally toothed gear such that relative motion between the gears is created. This relative motion can be realized as a gear ratio. The end result is a speed/torque trade off that has high value in the motion control market.
A tubular shaft was added to the externally toothed gear (flexspline) to achieve many of the features of strain wave gear technology. The tubular shaft allows the strain wave gearing to be zero backlash, decreases bearing loads, and balances internal forces. It also dramatically increases the strain life of the externally toothed gear by distributing the strain over a longer distance.
Strain wave gearing has multiple uses. One use is as an integrated gear system designed in a specific machine for a specific purpose. These systems are highly engineered and customized for a particular application. Additionally, a strain wave gear set can be configured into a housing with an input and an output to be utilized by another user, typically referred to as a gearbox. These gearboxes are configured more for the general market, where an integrator would pair it up with other components to build a machine. Strain wave gearboxes come in many forms but have some things in common. First, they have an input, either a shaft, a flange, or a bore. They also contain an output in one of the same three options. Furthermore, they include a housing and some combination of bearings.
Installing the elliptical member, also called the wave generator, into the flexspline is a critical step in obtaining the proper performance of the gearset. One manner of installation is to attach the wave generator to the input and install as an assembly. There are multiple disadvantages with a system in this configuration. First, the input needs to have a custom modification to properly position the wave generator, such as a bolt and step. This custom modification can add significant costs to the system. Second, the end user is the one ultimately responsible for properly positioning a critical component of the gear system which creates risk for the end user. If the positioning can be done by the manufacturer, control of the precise position of the system is assumed by the manufacturer rather than the end user, ultimately increasing the product performance.
Another method of installation is to have the wave generator constrained, then install the input. As an example, the wave generator has been constrained by using ball bearings positioned on one or both sides of the wave generator. This constraining method allows the manufacturer to properly position the wave generator instead of the end user. By doing so, the end user just needs to connect to the system with a simple coupling device, such as a key, a bolted connection, a clamp collar, bolts or the like.
This method has a disadvantage because the ball bearings over constrains the system radially, such that any error in the manufacturing of the bearings, or the parts the bearings are attached to, will load the bearings in an undesirable manner. Each of the three bearings will have different centerlines, which is the case in any manufactured part simply due to machining tolerances. When the shaft is rotated, the eccentricities create radial loads in the bearings that are a function of the amount of eccentricity in the system.
One of the largest advantages of strain wave gearing is its size compared to other gearings systems, such as planetary gears. Strain wave gears are significantly smaller in size as that of other gearing systems with similar ratios. Reduction in the size of a strain wave gearbox further increases the value of that product.
The input connection is a feature that can be improved on to reduce the length of the gearbox, therefore adding performance. The input connection can be done in many ways including, a keyed connection, a friction lock, a taper lock, fastener connection or the like. In conjunction with the connection, there is usually a compliance device used to compensate for misalignments between the rotational axis of the input and the rotational axis of the wave generator. In some cases, no compliance device was provided for the bearing on the wave generator, but this method of connection risks damaging the wave generator bearing due to eccentric loading. The typical compliance connection used in strain wave gearing is an Oldham style coupling. An Oldham coupling uses two 90 degree opposed drive lugs to transmit torque. Those drive lugs are connected via a floating member, thus allowing for compensation of axial misalignment. The disadvantage is that the Oldham style coupling adds length and backlash to the system. The backlash comes from the need to allow room for the drive lugs to slide radially to compensate for parallel misalignment. The length is simply due to the fact that the Oldham style coupling is positioned axially next to the wave generator.
These systems also require lubrication and, thus, need to be sealed. In order to provide sealing, typical designs use methods such as O-rings, gaskets, or joint sealant. Each of these methods has a disadvantage. Specifically, O-rings require significant space, resulting in a larger product; gaskets add length to the system and create a flexible member between two joints, which decreases the overall system stiffness; and a joint sealant is difficult to apply in a consistent amount over the full connection, creating a flexible member between joints, risking not having sealant at portions of connection, and allowing for leakage.
Various types of bearings can be used to support the output. Most bearings, such as cross roller bearings, need to be lubricated before use, and periodically over the product life. The cross roller bearing has provisions located on the outer race in the form of radial holes to be used for re-greasing. Typically, re-greasing is done by the end user by applying a grease gun to a fitting installed by the gearbox manufacturer. However, customers do not like to use grease as it is messy and can contaminate surrounding items; it is difficult to ensure the proper amount of grease was added; and it can be difficult to get to re-greasing points.
Thus, a need exists for methods and systems which overcome the deficiencies of the prior art.